Regulations and codes, such as the ASME Boiler and Pressure Vessel Code, require that pressure relief valves be serviced periodically to warrant and check for the correct set pressure, mass flow rate and functional stability. Servicing of pressure relief valves is typically conducted one to two times a year and often mid to large sized facilities have in excess of 200 to 1,000 pressure relief valves operating within their system. Hence, it is evident that a large amount of labor is required in complying with the regulations and codes. Of course, if such regulations and codes are not followed, catastrophes involving the release of toxic fumes such as that which occurred in Bhopal, India can result. Also, since having to shut down a facility to service pressure relief valves is ecomonically detrimental, it is desirable that such servicing occur without process interruption.
The prior art devices presently being utilized for the servicing of safety and pressure relief valves without process interruption feature the installation of two redundant safety relief valves. In general, three different valve systems have evolved in attempting to achieve servicing of pressure relief valves without process interruption. The three systems generally relied upon are as follows:
(1) A system with two separate vessel penetrations with mechanically linked block valves; PA1 (2) A system with a 3-way block valve; and PA1 (3) Safety selector valves with a movable isolation disk and a single inlet and outlet.
The above noted prior art designs require, in addition to a redundant pressure relief valve, additional piping fabrications and additional block valves or switch-over valves. The above noted prior art systems are relatively costly and also lead to various problems such as (1) high inlet pressure drop (which in turn leads to valve instabilities), (2) high maintenance requirements, (3) reliability problems associated with the valve system's relatively complex systems, and (4) the problem of the combination discharge bursting disk and valve discharge capacity.